No one will face charges over the wrongful conviction of Stefan Kiszko, who served 16 years for a child murder he did not commit, because of the death of the detective who headed the inquiry.

A magistrate has thrown out charges of perverting the course of justice against another officer and a forensic scientist after deciding that they could not get a fair trial without the evidence of Detective Chief Superintendent Jack Dibb.

Jane Hayward, stipendiary magistrate at Rochdale, said that retired Superintendent Richard Holland, 62, and former forensic scientist, Ronald Outteridge, 68, may be in the dock "for acts or omissions which in reality are those of Mr Dibb". Alternatively, she said, he may have made statements to them "which would excuse any criminal liability on their part".

Yesterday the Crown Prosecution Service decided not to challenge her decision, prompting calls for a public inquiry, because the ending of the criminal case means that what caused one of the country's gravest miscarriages of justice will remain shrouded in secrecy. The Court of Appeal which freed Mr Kiszko in 1992 heard that forensic tests taken at the time of his conviction - but not revealed to the court or the defence - proved then that he could not have been the killer. Now it may never be known exactly how that information lay buried for 16 years.

Campbell Malone, Mr Kiszko's lawyer, said: "Stefan and his mother went through an awful ordeal and no one knows why. There now needs to be a public inquiry where witnesses are compelled to give evidence so that we can get at the truth, so that public confidence can be restored and so that important lessons can be learnt."

Mr Kiszko was jailed in 1976 for killing 11-year-old Lesley Molseed, who was found stabbed to death on moorland near Rochdale. Mr Kiszko, who had a mental age of 12, was said to have confessed to the crime. But tests on semen stains on her clothing revealed that he could not have been the killer.

A rare condition had left him unable to produce sperm - Lesley's murderer had left sperm traces on her clothing. But this was not revealed at his trial and only came to light in 1991 when West Yorkshire police were reviewing the conviction. Mr Kiszko died in 1993 a year after his release, while his mother, who had campaigned for his freedom, died six months later.

After further investigations, Mr Holland, second in command of the initial investigation, and Mr Outteridge, principal scientific officer, were charged. The magistrate said that the prosecution alleged they decided to deliberately exclude from the evidence any reference to the two conflicting test results. In her ruling, she said Mr Holland maintained that if the laboratory had told him the samples did not match, Mr Kiszko would not have been charged. Mr Outteridge said he had no doubt he had told the investigating officers that Kiszko's sample contained no sperm heads and the differences between the findings was discussed.

The ruling was made last month, but was made public by the Attorney General, Sir Nicholas Lyell, in a letter to Chris Mullin, the Labour MP, after the CPS decided not to challenge her decision by judicial review.